Civic art project: 5'H x 5'W 3D heart shape sculpture. Can Blender map a 12" grid on it? From photos?
Yes. 

You will first have to model the heart which shouldn't be a problem considering you can get all the dimensions you need.
It may take a lot of tweaking to get precise measurements but you can get very close with little difficulty.

Blender has an option to change units. This includes Inches :D

Once you have created the model, look into UV unwrapping in Blender. There is plenty of tutorials on it. You can paint the object in Blender without exporting but I recommend exporting it and using a program like Gimp or Photoshop for better results. You could also print the UV and draw over it, then scan it back into the program.

You should be able to find tutorial videos on all the things you wish to do.

Some other helpful tools you may want to look into would include:


Grease pencil
Mirror Modifier


Helpfull Links:


Units in Blender
Blender for Beginners
UV Unwrapping


Though it won't make much of a difference when modelling, if you want to render a design picture, I recommend using cycles engine.

Good luck with your project and I hope I helped,

BFB